var base64Str = 'emhhbmdzYW46MTIzNDU2';
base64Str=Buffer.from(base64Str,'base64') ;
var utf8Str=base64Str.toString('utf-8');
var utf8Arr=utf8Str.split(':');
var str="用户名:"+utf8Arr[0]+" 密码:"+utf8Arr[1];
console.log(str);